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Normal composites are the materials which just embed one or more materials in another homogenous matrix material, the phases of them are discrete, dispersed or isolated . In other words, the phases are not interconnected. The models in Figure 1 to show the different levels of connectivity as defined in a two phase composite, for example the continuous fibers reinforced composites have single connectivity (1-3) whilst the laminated materials are sheet connectivity (2-2) in the figure 2. In another word, most of them are anisotropic. That means, many properties of them are influenced by the direction of their structure, for example, there are 21 at most independent elastic constants are needed to know if their elastic behaviors want to be defined .
Figure 1 Model of connectivity in ten different levels, which defines in a composite with two phase 
Figure 2 Types of composite based on the form of reinforcement 
The interpenetrating composites are the materials which both matrix and reinforcements are three dimensional (3-3) throughout the microstructure, i.e. they display 3-3 connectivity. It is possible to produce such composites by infiltrating porous ceramic with molten metals. Examples of porous alumina are shown in Figure 3 whilst the microstructure of an alumina/aluminium-magnesium alloy interpenetrating composite (IPC) is shown in Figure 4. It is difficult to say most of them are isotropic but at least their properties are approaching to the isotropic body.
Figure 3 SEM micrograph of the used Al2O3 foam in the research of Hong. 
Figure 4 Optical micrographs of (a) the uncompleted Al2O3 foam / Al-10%Mg infiltrated composite (b) the completed Al2O3 foam / Al-10%Mg infiltrated composite 
Potential of metal ceramic interpenetrating composites
The three fundamental kinds of materials are metal, ceramic and polymer. Each of them has own advantages and disadvantages. Most metal materials have good properties of engineering. However, most of them have a high density and are limited by their fusion temperature in new technology. Ceramic possess medium density, high hardness, high temperature capability and very well chemical and environment stability, but their brittle is a critical defect in engineering application. Polymers are of low density. They are easy to fabricate but poor at engineering properties and a low stability of thermal and environment.
For improved performance of materials in every area, there is a need for improved properties, therefore composite which contains properties of two different materials, have been developed. Since most of they have superior mechanical properties compared with the monolithic materials in relative application, they have became widely used in all world, especially in structural materials .
By reason of the isolated phases of microstructure of traditional composites, there are some disadvantages such as the properties are limited by volume fractions of different phases, direction of their structure or influenced by the dominated matrix. So a more developed material is needed to overcome these drawbacks and many simple models show that the properties of composites, which are normally formed with stiff and complaint materials, will increase when the phases become continuous . Hence, there has been an interest in the interpenetrating composites with the superior multifunctional properties which they are provided and the so widely practical and potential applications .
The superior properties of metal ceramic interpenetrating composites
Flexural strength and toughness
Most of present metal-ceramic interpenetrating composites are Al based composites, to compare with ceramics, they can provide superior tolerance of damage and facture toughness. It can be known from Figure 5.
Figure 5 Flexural strength of: (a) the un-infiltrated Al2O3 foams, and (b) the Al-8Mg/ Al2O3 composites .
To compare with Figure 5 (a) and (b), it clear that the materials infiltrated with alloy has a much higher flexural strength than the ones without infiltrating. The reason is that the reinforcement, metal or alloys, in composite forms many ligaments, hereby there is a crack bridging to reduce the tip stress by preventing the crack wake when an external force or damage on the materials [3ï¼Œ7, 8, 9, 10, 11]. The Figure 6 gives the possible schematic diagram about reinforcing mechanisms. There are two main ways to form crack bridging in order to reinforce the composites, one is formed by ductile phases (reinforcement) and anther is formed by matrix grains. In addition, crack deflection and process zone shielding are also possible ways to reinforce composites [3, 12, 13, 14, 15].
Figure 6 Diagram of two possible crack bridging mechanisms in metal ceramic composites: (1) bridging by ductile phases and (2) bridging by matrix grains .
These mechanisms can be proved in the research of Hong  of Al-8%Mg/ Al2O3 composites, which are showed in Figure 7. The crack is obviously deflected by forming crack bridging of the matrix in Figure 7 (a), and in Figure 7 (b) the crack is prevented by the ductile reinforcement.
Figure 7 Back-scattered electron micrographs of the aluminium-8Magnesium/alumina composites (a) deflection of crack propagation route; (b) break crack by reinforcement .
3.2 Wear resistance
Interpenetrating composites are also promising in providing wear-resistant property, according to the study of Ceschini et al.  about interpenetrating Al2O3/Al composites. It is found that a scar depth in conventional MMCs is ten times higher than the scar on interpenetrating composites. In the research of Hong , the same situation is found from a group data of wear rates of the aluminium-magnesium alloy and the aluminium-8magnesium/alumina composites in Figure 8 and Table 1. From these data, the wear rate of Al-Mg/ Al2O3 composites increased with the ceramic matrix density and the size of the cell decreased. The observed lowest wear rate, 1 - 10-12 m3/m, is of the composite with the largest cell size of 160 μm and highest foam density of 27%.
Figure 8 The wear rates of (a) the aluminium-magnesium alloy and (b) the aluminium-8magnesium /alumina composites under different loads.
Table 1 The wear rates of the aluminium-8magnesium alloy and the aluminium-8magnesium/alumina composites under the same load .
There has not a certain behavior of wear-resistant because of wear is a complex process, the relative factors contain testing conditions, characteristics of matrix and reinforcement, interfacial bond of matrix-reinforcement [3, 18].
3.3 Thermal and elastic properties
Over a certain range of temperature, there is a coefficient relate the temperature interval and thermal strain, this is called coefficient of thermal expansion (CTE) . It is well known that the CTE value of ceramic is low and the CTE value of metal is much larger. It is reported the CTE of metal-ceramic composites is similar with ceramic-matrix composite, and the CTE value of ceramic-matrix composite is low in metal-ceramic composite by a study of Shen , which can be known from Figure 9. In addition, unlike traditional composites, interpenetrating composites have a superior resistance to thermal cycling damage from a study on Al-Al2O3 and NiAl-Al2O3 composites [3, 20].
Figure 9 Different CTE curve of composite with different volume fraction of SiC .
According to the research of Hong , there is a significant improvement of hardness in interpenetrating composite. The Table 5 shows the data of hardness under Rockwell test. Practice has proved that the values of hardness between various materials have an approximately corresponding relationship with the values of strength. Because the hardness value is determined by the starting plastic deformation resistance and the decision to continue plastic deformation resistance, the higher the strength of the material and plastic deformation resistance, the higher the value of hardness.
Table 2 Hardness of the alloy and the interpenetrating composites .
3.5 Ballistic property
As well known, the high penetration resistance, the capability of the multi-hit potential and the lower weight are needed by kinds of ballistic applications. According to these conditions, many ceramic materials are considered by their abrasion resistance and moderate weight. However, most ceramics are brittle, i.e. they have a poor multi-hit potential, therefore they cannot stand several penetration and then need to be replaced [3, 21]. Even there is a report about Ti 6-4 backed ceramic, which a ceramic-based core is surrounded by a Ti-6-4 layer, shows some improvement in the ballistic test but it has no significant improvement than the monolithic material according to another study about TiB2/TiB/Ti functional materials with several layers [3,22]. Thereby there has been an interest in the interpenetrating composites for this aim. The investigation about the ballistic property of aluminium-magnesium/alumina interpenetrating composite is reported in the research of Hong , the Figure 10 shows the schematic of the ballistic test.
Figure 10 Schematic of the ballistic test in the research of Hong 
It is reported that there are 5-6 mm reduced in the penetration depth of the target with composite and a deflection in the penetration direction, the penetration depth in the Al target had been pictured and measured, which are shown in Figure 11 and Table 3.
Figure11 Micrographs of the ballistic tests: (a) pure aluminium target; (b) with the
aluminium-magnesium /alumina composite layer.
Table 3 Penetration depth in target Al, in mm.
The route of making metal ceramic interpenetrating composites
Most present processes of interpenetrating composite are infiltration processes. There is a category by Mortensen  of produce routes of metal-ceramic interpenetrating composites, they are liquid state, solid state and deposition process. Amongst them, the liquid state is widely used by reason of good bonding between matrix and reinforcement, wide section of materials and advantage to get a near-net shape in relatively structure [3, 24]. The liquid-state infiltration processes are further divided into forced infiltration and pressureless infiltration by considering the external pressure.
There are several ways of forced infiltration by different sources of the force or pressure . For example, vacuum infiltration, gas pressure infiltration, pressure assisted investment casting and squeeze casting infiltration. There is a direct squeeze casting schematic in Figure 12. In this process, before the molten aluminum is poured onto the preform, the die and preform are preheated. Then infiltration is occurred under the pressure of upper punch and after that is solidification [3, 25].
Figure12 Schematic of a direct squeeze infiltration process .
There are several advantages of forced infiltration, such as a high product rate result from the high infiltration rate and a high density of product result from the extra pressure. However, the disadvantages are also obvious. The size of die due to a small size and the shape of the product is also hard to be complex. Beside these, the high extra pressure, the more soft materials will be crushed.
4.21. Reactive melt infiltration
To compare with forced infiltration, there are few ways to produce interpenetrating composites by pressureless infiltration. Reactive melt infiltration is one, there is a reaction between a sacrificial preform and a molten metal to drive the metal into the preform [3, 26, 27, 28]. Figure 13 shows one reaction and growth mechanism of an Al/ Al2O3 composite, the sacrificial SiO2 preform is immersed into the molten Al, then the reaction between them make the SiO2 transform into the interpenetrating composite.
Figure 13 A schematic of the growth mechanism of the Al2O3/Al composite by reaction of SiO2 with molten Al .
By reason of the kinetics of reactions between the sacrificial perform and the molten metal, the infiltration rate of this process is low [3, 30]. However the infiltration rate can be changed by adjusting the parameters which include temperature, pressure and the components of the sacrificial perform.
4.22 Pressuresless infiltration Process of Hong 
Hong  did this research about using an alumina foam and an aluminium-magnesium alloy to make the Al-Mg/ Al2O3 interpenetrating composite and investigate its properties. The alumina/ aluminium-magnesium interpenetrating composite is made from infiltrating the aluminium-magnesium alloy into the aluminium foam. In this research, another technique is used to produce metal ceramic interpenetrating composite. In this process, an Al2O3 crucible which loads Al2O3 preform and Mg-Al alloy is put into a furnace tube, which is shown in Figure 14. The schematic of this process is shown in Figure 15. The furnace tube is fixed in the furnace and heated to 915°C with full atmosphere of Argon, at 915°C the air atmosphere in the tube is changed to Nitrogen in order to get a good infiltration. After about half an hour, the tube is begun to cool and at about 700°C, the air atmosphere is changed back to Argon. The equipment of this process is shown in Figure 14.
Figure 14 Schematic of the metal-ceramic alloy, Al2O3 foam and Al2O3 crucible .
Figure 15 Schematic of the furnace setup .
In this process, the shape of the product has a large improvement and the size of product also has some growth according to the size of tube. While to compare with the widely used forced infiltration processes, the infiltration rate is much lower.
5. The problem of metal ceramic interpenetrating composites
According to the properties of varied metal ceramic interpenetrating composites which are reviewed above, they are proved that there are few problems in properties of interpenetrating composites. However, there is a large problem in producing process, it is the weak wettability between most metals and ceramics.
5.1 Wettability between a solid and a liquid
Wettability defines the extent to which a liquid will spread over a solid surface. There is an association between the material surface and the free energy. There is an illustration of a liquid drop which has been allowed to reach equilibrium in Figure 16.Only when the viscosity of matrix is not too high and the free energy of the system decrease with wetting, the wetting will occur. The surface tension can be shown to instead of the free energy of an interface, therefore according to Young's equation, the basic relationship of wettability, at equilibrium, can be given as
γsv =γsl + γlv (5.1)
Where γsv :the solid-vapour interface energy,
γsl : the solid-liquid interface energy,
γlv :the liquid-vapour interface energy,
θ: the contact angle.
Rearranging this expression gives
=(γsv-γsl)/ γlv (5.2)
Θ usually is used to measure of the degree of wettability. For a contact angle of 180°, the drop is spherical with only point contact with the solid and no wetting takes place. When θ=0°, it gives perfect wetting. For intermediate values of θ, the degree of wetting increases as θ decrease. Usually it is considered that liquid wets the solid if θ<90°.
Figure16 A liquid in equilibrium with a solid with a contact angle 
5.2 Infiltration dynamics of cylindrical capillary
It is a familiar phenomenon about a liquid rise or fall inside a cylindrical capillary tube which is put vertically into a liquid like Figure 17 shown.
Figure 17 Capillary rise and depression in cylindrical capillaries in a liquid .
The reason of this phenomenon is the different wettability between the liquid and solid (the capillary tube). The capillary force leads the liquid rise when the contact angle θ < 90°, the liquid wets the solid. When the contact angle θ>90°, the liquid does not wets the solid, the liquid fall in the tube. In addition, the relation of gravity and the capillary pressure is:
Pc = ==ρl g h (5.3)
Where, Pc : the capillary pressure;
r : the capillary radius;
ρl : the liquid density;
g : the gravitational acceleration
h : the height of the liquid inside the capillary.
5.3 Infiltration kinetics
In the model which is proposed by Martins et al.  about Spontaneous infiltration of metal and porous preform, there are four acting forces on the liquid which is inside the cylindrical capillary tube. They are gravitational force, Fzg ( Fzg=-πρgh 5.4) , surface tension force, Fzr( Fzr =2πrγlv cosθ 5.5), poiseuille viscous drag force, F zμ (F zμ=-8πμh 5.6), where, μ is the viscosity of the liquid), and end-drag force, Fze (Fze=-πρ 5.7).
According to these forces and their expressions, the infiltration rate parameter,φhe the dynamiccan be given as:
φ=(rγlv cosθ)/2μ (5.8)
and then the relationship between the infiltration time, t and the infiltration height, X is given
So the factors which influence the infiltration kinetics/rate include the capillary diameter, r, the contact angle, θ, and the viscosity of the liquid, μ.
5.4 Methods of improving wettability
To solve the weak wettability between most metal and ceramic, several wetting-improving methods, which include metal alloying, ceramic/solid particle coating and treatments, were reviewed by Delannay et al. .
There are three ways to improve the wettability between metal and ceramic by adding another alloying element: to reduce the free energy of liquid surface, to decrease the interfacial energy of solid-liquid and to promote the solid-liquid interfacial reaction [3, 35]. In the process of Hong , Mg is added as an alloying element, it acts one important role is that to prevent the reaction of Al and residual by reacting with the residual first. Because the reaction of Al and can form a protective oxide layer around the metal and to prevent the infiltration but the reaction of Mg and can form a non-protective oxide .
The wettability of most ceramics by molten metals can be improved by coating the ceramics which have a weak wettability by metal with a metallic layer result from the surface energy of the solid increases .
It is reported that wettability of ceramic between Al can be improved when the ceramic particles are pre-treated using K2ZrF6 [3, 37]. Beside these, to clean the particle surface and to add the additives are also useful to improve the wettability between some metals and ceramics. In the process of Hong , the role of is an additive. It reacts with Mg to form , then the react with Al to form AlN, the AlN have a much better wettability of Al than Al2O3. The relative reactions is shown in Figure 18.
Figure 18 Schematic of role of N2 
Now metal ceramic interpenetrating composites are only achieved in few materials and there is no a certain theoretical system to explain the behaviors of them. Most work is focused on the characterisation of these materials in recent years. However, there are still some achievements, especially in the knowledge of improving wettability between Al and Al2O3. A good quality infiltrated product is achieved by controlling relative factors.